This invention relates to a method and apparatus for applying a secondary filter layer or precoat layer in a plate filter press. As is well known, in order to apply a precoat layer to the filter cloths of a filter press, a suspension of the precoat medium and a carrier medium is fed to one or both ends of the filter press and the carrier medium is drained off.
It is known that in the filtration of sludges in plate-type filter presses the separation of the filter cake from the filter cloth presents difficulties with various types of sludge. For example, these types of filter cakes include filter cakes produced from tailings in the production of coal, filter cakes produced from sludges flocculated with polymeric flocculating agents, and the like. The adhesion of the filter cake to the filter cloth necessitates disagreeable manual labor, which is undesirable and uneconomical.
In order to reduce the adhesion of the cake to the filter cloth sufficiently for the fall of the cake to occur automatically and without residue, it is necessary to apply precoat layers to the filter cloths. Heretofore, this has been done in such a way that, after closing the filter press but before the actual filtration charge, a suspension of a precoat medium or also of a framework-forming secondary filter medium is fed to the chambers of the filter press and the filtrate passing through the filter cloths is drained off, so that the precoat medium is precipitated upon the filter cloths.
However, it has been discovered that, especially with large filter presses, such as with a filter plate of a size ranging from 1.5.times.1.5 m and having more than 50 filter plates, it is extremely difficult to distribute the precoat layer uniformly over the total filter surface and to ensure that a precoat layer is applied at all regions, particularly in the top regions of the filter cloths. With filter presses of great length, and insufficient application of precoat layer is observed, particularly in their central regions, even though the charging with the precoat suspension is performed from both ends of the filter press.
In such cases, if it is attempted to continue the feeding of the precoat suspension until a sufficient precoat layer has been deposited in the central part of the filter press, and in the top regions of the filter cloths, then a very large quantity of liquid and a very long time is required to deposit the precoat layer. Also, during this time a thicker precoat layer than is necessary to eliminate the filter cake adhesion will have been deposited in the bottom regions of the filter cloths and particularly in the end sections of the filter press. The consumption of precoat is therefore unnecessarily high, and the filter resistance of the filter cloths thus coated too thickly with precoat is increased unnecessarily.